Vagal control of the rat cranial venae cavae

The cranial venae cavae of the rat heart are composed of cardiac muscle from the atria to the root of the subclavian veins (Endo et al. 1994). An investigation was carried out to test whether the vagus nerve has an inotropic action on these blood vessels. Male Wistar rats (weighing 220-300 g) were killed humanely and the heart was rapidly removed. An atrial vena cava preparation was maintained viable at 37 °C by superfusing it with warm filtered Hepes-buffered Tyrode solution (pH 7.4) bubbled with 100 % oxygen. Either the right or left vena cava was pinned out on Sylgard and the rhythmical contractions recorded with a hook attached to a force transducer. The ipsilateral vagus or cardiac branch was stimulated with a glass suction electrode acting as a cathode (10-20 V, 1 ms, 30 Hz). Stimulation of vagal fibres (n = 5 animals) produced a significant negative chronotropic and inotropic effect. Before stimulation the basal cardiac interval was 317 ± 26 ms and the vena caval diastolic force was 2 ± 0.26 mN and the systolic force was 3.56 ± 0.5 mN. Ten second stimulation increased the cardiac interval to a maximum of 484 ± 77 ms and reduced the systolic force significantly to 3.07 ± 0.4 mN (P = 0.0075, two-tailed paired t test). The diastolic or baseline force was unaffected by vagal stimulation.

The vagal negative inotropic action took significantly longer to reach peak effect (10.2 ± 0.6 s versus 3.3 ± 0.6 s) and lasted longer than the chronotropic effect (22.9 ± 1.3 s versus 9.6 ± 0.8 s). The negative inotropic action was still observed in paced preparations. Both the chronotropic and inotropic effects were abolished by atropine (10-6M; n = 4 animals) or hexamethonium (1 mM; n = 2 animals) and mimicked by acetylcholine chloride (10 nM). In order to minimize an atrial contribution to the force production a more reduced preparation was utilized and ganglion clusters at the cavo-atrial junction were stimulated electrically. Similar negative inotropic and chronotropic effects sensitive to hexamethonium were seen (n = 6 animals). After hexamethonium administration positive inotopic and chronotropic effects were uncovered and these were abolished by atenolol (0.1 mg% n = 4 animals). Methylene blue staining of the preparation at the end of the experiment showed the presence of ganglion cells at the sites of stimulation. Ganglion clusters were never seen on the venae cavae per se. The results of this investigation show that the vagus has a powerful action on the venae cavae resembling that on the atria. All values are expressed as means ± S.E.M.